Roasted coffee grains to be used in a beverage ready for consumption, such as espresso or American coffee, must first be ground, that is, processed in powder form, to better interact with the water passing through the coffee for the extraction of the beverage.
The preparation of coffee, in particular of an espresso coffee, which meets the consumer's taste depends on many factors, among which the grain size of the coffee powder plays an important role. In the traditional preparation of espresso coffee, for example in coffee bars and restaurants, the coffee is purchased in grains and crushing of the grains is made on site by means of a metering device, which carries out the grinding, dispensing ground coffee. The degree of grinding and its control are often left to the experience of the bartender, who makes corrections in case of change of the product in grains and/or of the quality of the dispensed beverage.
In the large-scale retail trade, coffee is often sold in vacuum packages or cans of ground coffee or in the form of capsules or pods ready for use. An analysis of the degree of grinding of the marketed product may be fit or desired for quality control of the product both by the producer and by the purchaser, or to know the characteristics of the product.
To this end, analysis techniques have been developed for the measurement and analysis of the grain size of coffee powder. In general, a characteristic among the most studied is the grain size of the particles of ground coffee. The grain size is typically controlled by determining a distribution curve of the ground coffee as a function of the size of the particles forming the coffee powder, usually indicated with granulometric curve.
A known, fast and inexpensive method is sieving, in which a series of sieves is used with gradually narrower meshes, the sieves being arranged vertically stacked. A predetermined amount of coffee powder is put on the top sieve and all of the sieves in the series are vibrated. The powder collected on each sieve is then weighed. However, the result depends on the amount of powder used, on the duration of the vibratory motion, on the number of sieves used and on environmental factors that modify the aggregation of the particles in the coffee powder (humidity, for example).
The size of the coffee particles can be directly estimated by an optical microscope. A sample of powder is distributed on a slide on which a grid is drawn that indicates the micrometer scale. In addition to problems related to the representativity of the analysed sample, which could be statistically not significant, the evaluation of two particle sizes is only possible with this technique, namely the length and width measured in an optical plane.
A method increasingly used is laser diffractometry, which uses a laser beam at low power in the visible or in the near infrared. The laser beam passes through a sample of coffee powder contained in a cell so as to generate a diffraction pattern which provides information about the particle size distribution. The size range possible with this technique is typically comprised between about 1 and 103 microns. The powder inside the sample can be dispersed in suspension or being in a dry dispersion. The laser diffraction technique is based on a “cumulative” measurement of the sample to obtain the percentage volume of particles having a certain size.
Recently, studies have been published on the microstructural properties of coffee beans (not ground) by using X-ray microtomography. Paola Pittia et al. in “Evaluation of microstructural properties of coffee beans by synchrotron X-ray microtomography: a methodological approach”, published in the Journal of Food Science, vol. 76 (2011), pages 222-231, describe the use of microtomography based on synchrotron X-ray as a non-destructive imaging technique to study microstructural properties of raw or roasted coffee beans. 2D images were reconstructed with this technique and 3D images of the grains were obtained, which were used to calculate and quantify the porosity of the grains.
P. Frisullo et al. in “Coffea arabica beans microstructural changes induced by roasting: An X-ray microtomographic investigation”, published in Journal of Food Engineering, vol. 108 (2012), pages 232-237, show 3D images obtained by X-ray microtomography of whole beans of roasted coffee. The authors state that from the 3D image analysis it was possible to derive the size, shape, distribution of the total volume, porosity and density and to quantify the structural alterations of the microstructure caused by the high internal pressure generated during the thermal treatment of roasting.
The interaction between an atomized fluid and a solid surface and in particular of a single droplet deposited on a surface was studied by X-ray computed microtomography (microCT) in “X-ray computed microtomography for drop shape analysis and contact angle measurement”, published in the Journal of Colloid and Interface Science, Vol. 409 (2013), pages 203-210, by M. Santini et al. The three-dimensional surface of the drop was reconstructed to perform contact angle measurements on the actual cross-sections of the drop-surface pair, with a resolution of about 9 μm.
M. Santini e M. Guilizzoni in “3D X-ray Micro Computed Tomography on Multiphase Drop Interfaces: From Biomimetic to Functional Applications” describe three cases of analysis by means of interaction multi-phase microCT, two are microCT of a sessile drop of water on a leaf and the third case is a sessile drop of water on an artificial surface, i.e. a layer of material adapted to the gaseous diffusion (GDL).